1. Field of the Invention
The present invention relates to the use of special UV-curable coating compositions based on urethane-modified acrylate resins for coating molded articles of thermoplastic polycarbonates.
2. Description of the Prior Art
Polycarbonate molded articles are used in a variety of ways because of their excellent properties, such as transparency, impact or shock resistance and tensile strength. However, the properties of the surface of the material, such as its low abrasion and scratch resistance and the low resistance to solvents, are inadequate for many areas of application.
In the past, very different solutions have been proposed for the removal of this drawback (cf e.g. PCT Applications WO 80/00968 and 86/04592, U.S. Pat. No. 3,968,305, EP-A 0,274,595 and EP-A 0,274,596). These publications are primarily directed to the coating of molded articles with UV-curing lacquer systems. The processes of these prior publications, however, have a number of serious disadvantages. In particular, the adhesion of the lacquers or the scratch resistance obtained with the lacquers, as the case may be, is not completely satisfactory. The coating compositions of the two last-mentioned prior publications cure only under nitrogen as a deactivating medium.
The coating compositions based on polyacrylate resins having urethane groups according to EP-A 0,020,344 or according to PCT-Application WO 80/00942 also lead to coatings that still do not fully satisfy the practical requirements with regard to scratch resistance and stability to weathering.
The coating compositions according to DE-OS 3,134,157 contain fairly large amounts of N-vinyl derivatives of linear or cyclic amides. A disadvantage of these compositions is the need to carry out the UV irradiation several times in order to obtain good scratch resistance. Furthermore, a low viscosity can be obtained in this case only by using large amounts of N-vinyl pyrrolidone. However, this reduces the resistance to scratching and to solvents as well as the stability to weathering. In addition, if fairly large amounts of N-vinyl pyrrolidone are used, the risk exists of the partial solution, whitening and stress cracking of the polycarbonate, as long as the coating is not yet cured. N-vinyl-pyrrolidone is also undesirable with regard to job hygiene.
The urethane acrylates according to DE-OS 3,819,627 also are used in combination with N-vinyl compounds, which leads to the disadvantageous consequences already mentioned.
The urethane acrylates according to DE-OS 4,021,109 are used in combination with tri- to hexafunctional (meth)acrylates as reactive diluents in order to avoid the said disadvantages of the aforementioned solvents. However, the stability to weathering of the resulting lacquer films is insufficient. This becomes recognizable as a loss of adhesion even after a relatively short time.
Finally, DE-OS 3,318,147 is concerned with special urethane acrylates without any connection to the special problem of coating polycarbonate molded articles.
It is an object of the present invention to provide coating compositions for coating thermoplastic polycarbonates which do not suffer from the disadvantages of the coating compositions of the prior art.
This object may be achieved with the coating compositions of the present invention which are described in more detail below. The coating compositions to be used according to the invention differ, e.g. from the coating compositions according to DE-OS 4,021,109 by the use of a selected low-viscosity polyisocyanate component having isocyanurate groups for the manufacture of the urethane acrylates as well as by the use of mainly linear bis-acrylates as reactive diluent B).